1. Field of the Invention
The present invention relates to techniques for dynamically adapting backlighting for displays. More specifically, the present invention relates to circuits and methods for adjusting video signals and determining an intensity of a backlight on an image-by-image basis.
2. Related Art
Compact electronic displays, such as liquid crystal displays (LCDs), are increasingly popular components in a wide variety of electronic devices. For example, due to their low cost and good performance, these components are now used extensively in portable electronic devices, such as laptop computers.
Many of these LCDs are illuminated using fluorescent light sources or light emitting diodes (LEDs). For example, LCDs are often backlit by Cold Cathode Fluorescent Lamps (CCFLs) which are located above, behind, and/or beside the display. As shown in FIG. 1, which illustrates an existing display system in an electronic device, an attenuation mechanism 114 (such as a spatial light modulator) which is located between a light source 110 (such as a CCFL) and a display 116 is used to reduce an intensity of light 112 produced by the light source 110 which is incident on the display 116. However, battery life is an important design criterion in many electronic devices and, because the attenuation operation discards output light 112, this attenuation operation is energy inefficient, and hence can adversely affect battery life. Note that in LCD displays the attenuation mechanism 114 is included within the display 116.
In some electronic devices, this problem is addressed by trading off the brightness of video signals to be displayed on the display 116 with an intensity setting of the light source 110. In particular, many video images are underexposed, e.g., the peak brightness value of the video signals in these video images is less than the maximum brightness value allowed when the video signals are encoded. This underexposure can occur when a camera is panned during generation or encoding of the video images. While the peak brightness of the initial video image is set correctly (e.g., the initial video image is not underexposed), camera angle changes may cause the peak brightness value in subsequent video images to be reduced. Consequently, some electronic devices scale the peak brightness values in video images (such that the video images are no longer underexposed) and reduce the intensity setting of the light source 110, thereby reducing energy consumption and extending battery life.
Unfortunately, it is often difficult to reliably determine the brightness of video images, and thus it is difficult to determine the scaling using existing techniques. This is because many video images are encoded with black bars, e.g., non-picture portions of the video images. These non-picture portions complicate the analysis of the brightness of the video images, and therefore can create problems when determining the trade-off between the brightness of the video signals and the intensity setting of the light source 110. Moreover, these non-picture portions can also produce visual artifacts, which can degrade the overall user experience when using the electronic device.
Hence what is needed is a method and an apparatus that facilitates determining the intensity setting of a light source and which reduces perceived visual artifacts without the above-described problems.